There are many sources of renewable energy that have been explored as possible means to limit the worldwide reliance on fossil fuels. Among the more promising renewable sources are solar energy, wind energy, hydro-generated energy (e.g., dams, tide-driven generators), geothermal energy, and biomass. With the exception of solar-derived, however, all of these sources suffer from inherent drawbacks. Hydroelectric energy, for example, requires massive infrastructure and along with it inevitable habitat destruction. Harnessing wind energy likewise requires considerable investment in infrastructure, namely large windmills arranged in very large arrays. Hydro energy, wind energy, and geothermal energy are also inherently limited to suitable geographic locations on the earth. Biomass, while promising, also brings with it potential interference with the production of food for humans because arable acreage is devoted to energy production rather than food production. The most readily available source of renewable energy, of course, is the sun. Even at the poles, the sun shines for at least part of the year. Solar energy can be harnessed passively, in the form of black bodies that are heated in the sun's rays (e.g, to heat water), or by using lenses that focus the sun's light to heat a given area. Solar energy can also be converted directly into electricity in a photovoltaic (PV) cell. The simplest of PV devices is a semiconductor photodiode. When photons of solar light contact the photodiode, electron-hole pairs (e−/h+) are generated, which are then used to carry an electric current.
Generally speaking, “photochemical” reactions are chemical reactions induced by light, e.g., photosynthesis. Photochemical reactions do not generate an electric current in the conventional sense. In contrast, photoelectrochemical (PEC) reactions operationally connect a semiconductor photovoltaic device with a chemical reaction such the energy of the photons striking the photovoltaic device are converted into electrochemical energy. The efficient use of natural sunlight in these reactions has been a long-standing research focus because PEC reactions are potentially more energy efficient than the corresponding reaction using a traditional catalyst.
For example, the idea of using a photoelectrochemical device to split water into H2 and O2 molecules has been investigated since the 1970's. In essence, a PEC semiconductor with appropriate electronic properties is immersed in an aqueous electrolyte and irradiated with sunlight. The photon energy from the sunlight is converted to electrochemical energy, which then breaks the H—O bonds in the water of the aqueous electrolyte. The fundamental atomic processes are reasonably well understood: Incoming solar photons of appropriate energy strike the PV cell and generate conducting electrons and corresponding holes, i.e, e−/h+ pairs. The electrons and holes move in opposite directions through the PV cell. In a simple, two-electrode device, the holes drive an oxygen evolution reaction at one electrode, and the electrons drive a hydrogen evolution reaction at the counter-electrode. See, for example, Chen, Zhebo; Dinh, Huyen; and Miller, Eric; “Photoelectrochemical Water Splitting, Standards, Experimental Methods, and Protocols,”© 2013, Springer-Verlag GmbH, Heidelberg, Germany, ISBN 978-1-4614-8298-7. See also Wenbo Hou and Stephen Cronin (2013) “A Review of Surface Plasmon Resonance-Enhanced Photocatalysis,” Adv. Funct. Mater. 23:1612-1619.
Similarly, photocatalysis is the acceleration of a photochemical reaction in the presence of a catalyst. Several groups have investigated using heterogeneous photocatalysts to drive industrially important reactions. See, for example, Phillip Christopher, Hongliang Xin, Andiappan Marimuthu and Suljo Linic (2012) “Singular characteristics and unique chemical bond activation mechanisms of photocatalytic reactions on plasmonic nanostructures,” Nature Materials 11:1044-1050. Here, the authors demonstrate ethylene epoxidation over an Ag/Al2O3 plasmonic metallic nanostructured catalyst. The authors conclude that this photocatalytic system exhibit fundamentally different behavior as compared to semiconductors. The photocatalytic reaction rates on these excited plasmonic metallic nanostructures exhibit a super-linear power law dependence on light intensity (rate∝Intensityn, with n>1), at significantly lower intensity than required for super-linear behavior on extended metal surfaces. Additionally, in contrast to semiconductor photocatalysts, photocatalytic quantum efficiencies on this plasmonic metallic catalyst increased with light intensity and operating temperature. See also Andiappan Marimuthu, Jianwen Zhang, and Suljo Linic (29 Mar. 2013) “Tuning Selectivity in Propylene Epoxidation by Plasmon Mediated Photo-Switching of Cu Oxidation State,” Science 339(6127):1590-1593.
Photocatalysis has also been investigated as a means to convert CO2 to hydrocarbon fuels (Cronin et al. (2011), “Photocatalytic Conversion of CO2 to Hydrocarbon fuels via Plasmon-Enhanced Absorption and Metallic Interband Transition,” ACS Catal. 1:929-936). Other groups have used visible light plasmonic heating of a gold/zinc oxide catalyst to drive a reverse water-gas shift reaction coupled to a methanation reaction. See Matranga et al. (2013) “Visible light plasmonic heating of Au—ZnO for the catalytic reduction of CO2,” Nanoscale 5:6968-6974. Photocatalysis has also been investigated in the context of using the forward water-gas shift reaction to generate hydrogen at room temperature. See Garcia et al. (2013) “Photocatalytic water gas shift using visible or simulated solar light for efficient, room-temperature hydrogen generation,” Energy Environ. Sci. 6:2211-2215.
In the patent literature, see US 2013/0122396, to Linic & Christopher (published 16 May 2013). The published patent application describes a plasmon-resonating nanostructure that catalyzes the reduction of an oxidant via a photo-thermal mechanism. The plasmon-resonating nanostructure can be a nanoparticle that comprises copper, silver, gold, or alloys these elements. The method is described as being useful to catalyze the reduction of an oxidant, for example, in a catalytic reactor or in a fuel cell. The only oxidant described, however, is molecular oxygen, O2. The publication describes CO oxidation with molecular oxygen as the oxidant (CO+½O2→CO2), as well as NH3 oxidation with molecular oxygen as the oxidant (NH3+O2→N2+N2O+NO+NO2+H2O [non-stoichiometric]).
Patent publication US2010/0288356, to Linic et al. (published 18 Nov. 2010), describes a composition comprising a semiconducting photocatalyst and plasmon-resonating nanoparticles. The plasmon-resonating nanoparticles are capable of concentrating light at a wavelength that is substantially the same as the wavelength of light necessary to promote an electron from a valance band to a conduction band in the semiconductor photocatalyst. Thus, the plasmon-resonating nanoparticles direct light to the band gap of the semiconductor at an increased intensity as contrasted to when the nanoparticles are not present.